The present invention relates generally to the field of fluid displacement apparatus, and more particularly, is directed to a scroll type fluid compressor having a variable capacity control mechanism.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 issued to Creux discloses such a device which includes two scrolls each having a circular end plate and a spiroidal or involute spiral element. The scrolls are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Since the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion, a scroll type fluid displacement apparatus may be used to compress, expand or pump fluids.
Scroll type fluid displacement apparatus are suitable for use as refrigerant compressors in air conditioners. In this application, thermal control in a room or control of the air conditioner is generally accomplished by intermittent operation of the compressor. Once the temperature in the room has been cooled to a desired temperature, the supplemental refrigerant capacity required of the air conditioner to maintain the room at the desired temperature need not be very large. Prior art air conditioners do not have capacity control mechanisms and therefore, supplemental cooling is provided by intermittent operation of the compressor. Thus, the relatively large load which is required to drive the compressor is intermittently applied to the driving source.
Prior art scroll type compressors which are used in automotive air conditioners are driven by the automobile through an electromagnetic clutch. Once the passenger compartment reaches the desired temperature, supplemental cooling is also accomplished by intermittent operation of the compressor through the electromagnetic clutch. Thus, the relatively large load which is required to drive the compressor is intermittently applied to the automobile engine.
It is, therefore, desirable to provide a scroll type compressor with a displacement or volume adjusting mechanism which controls compressor capacity as occasion demands, thus eliminating the need for intermittent operation of the compressor and the accompanying stress on the driving source and electromagnetic clutch. In a scroll type compressor, adjustment of capacity can be easily accomplished by controlling the volume of the sealed off fluid pockets. Such a capacity adjusting mechanism is disclosed in U.S. Pat. No. 4,468,178 issued to Hiraga et al. In the Hiraga et al. patent, the adjusting mechanism includes a pair of holes formed through the circular end plates of one of the scrolls. The holes are symmetrically placed so that the wrap of the other scroll simultaneously crosses over the holes. The opening and closing of the holes is controlled by valves.
In the Hiraga capacity adjusting mechanism, when the pair of holes is opened to effect a reduction in compressor capacity, fluid in the outer most fluid pockets is permitted to leak to the suction chamber through the holes. As fluid passes through the holes, there is a corresponding pressure increase in the suction chamber and a pressure loss in the outer most fluid pockets. As a result, the compressor capacity is not efficiently reduced. One proposed solution to this problem is to increase the number of holes formed in the circular end plate of one of the scrolls or to increase the diameter of the holes in order to enable easier flow of the passing fluid. It has been found, however, that such a solution is difficult to implement in practice and even further reduces the operating efficiency of the compressor.